Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are associated with long polyCUG and polyCCUG repeats in the 3′-UTR and intron 1 regions of the transcript dystrophia myotonica protein kinase (DMPK) and zinc finger protein 9 (ZNF9), respectively (Wheeler and Thornton 2007). While normal individuals have as many as 30 CTG repeats, DM1 patients carry a larger number of repeats ranging from 50 to thousands. The severity of the disease and the age of onset correlates with the number of repeats. Patients with adult onsets show milder symptoms and have less than 100 repeats, juvenile onset DM1 patients carry as many as 500 repeats and congenital cases usually have around a thousand CTG repeats. The expanded transcripts containing CUG repeats form a secondary structure, accumulate in the nucleus in the form of nuclear foci and sequester RNA-binding proteins (RNA-BP).
Several RNA-BP have been implicated in the disease, including muscleblind-like (MBNL) proteins and CUG-binding protein (CUGBP). MBNL proteins are homologous to Drosophila muscleblind (Mbl) proteins necessary for photoreceptor and muscle differentiation. MBNL and CUGBP have been identified as antagonistic splicing regulators of transcripts affected in DM1 such as cardiac troponin T (cTNT), insulin receptor (IR) and muscle-specific chloride channel (ClC-1).
Myotonic dystrophy type 2 (DM2) is associated with repeats in the first intron of the ZNF9 gene on chromosome 3. CNBP (ZNF9) is the only gene known to be associated with myotonic dystrophy type 2. CNBP intron 1 contains a complex repeat motif, (TG)n(TCTG)n(CCTG)n, and expansion of the CCTG repeat causes DM2. The number of CCTG repeats in expanded alleles can range from approximately 75 to more than 11,000, with a mean of approximately 5000 repeats.
DM1 and DM2 are associated with a variety of serious pathologies including muscle abnormalities and weakness, and in the heart, conduction abnormalities.